Installing firefox extensions/themes from files

Very often we have a .jar file for a firefox theme or extension and we don't know how to install it.
The proceure indeed is very simple. All we need to do is to have a look at
http://www.mozilla.org/projects/firefox/extensions/web-api.html

Although this refers to urls we can indeed specify a jar file using parameters of the form href="file:///absolute_file_path_here" and modifying the iconURL accordingly. Have a look at
www.cse.iitd.ernet.in/~avinash/install.html

I have also seen a firefox extension for the same purpose....but I don't have the link.